The invention relates to a matched filter, a spread spectrum receiver and particularly to the correlation of a reference signal and a signal incoming to the receiver.
A spread spectrum system is a telecommunications system in which the bandwidth used for transmitting a signal is substantially wider than is required for the data to be transmitted. The spectrum of the signal is spread in a transmitter by means of a pseudo-random spreading code, which is independent of the original data. In direct sequence spread spectrum systems (DS-SS), a spectrum is spread to the available bandwidth by shifting the phase of the carrier in accordance with a pseudo-random spreading code. The bits of a spreading code are usually called chips as distinct from actual data bits.
FIG. 1 shows a block diagram illustrating a direct sequence-based spread spectrum system. In the system, a signal of a data source 1-2 is first modulated in a data modulator 1-4 of a transmitter 1-1, whereupon a complex 1-6, 1-8 signal outcoming from the modulator 1-4 is modulated by multiplying said data modulated signal by a complex 1-12, 1-13 spreading code generated by a code generator 1-10 in a multiplier 1-14. A spreading code modulator 1-16 spreads the spectrum to be transmitted by means of the spreading code. A carrier generated by a high-frequency oscillator 1-20 is then modulated in a multiplier 1-18 by said data and code modulated signal, and an imaginary part 1-22 is removed from the signal to be transmitted. The transmitted signal propagates from an antenna 1-24 in the transmitter over a transmission path 1-26 to an antenna 1-32 in a receiver 1-30. In the receiver 1-30, a front end filter 1-34 separates an information signal from the entire frequency spectrum. A complex 1-35, 1-36 signal is mixed to a lower frequency by multiplying the signal by a complex 1-42, 1-44 signal generated by a voltage-controlled oscillator 1-40 in a multiplier 1-45.
In the receiver of a spread spectrum system, a reference signal, a code replica, which is an identical copy of said spreading code, is used in a despread modulator (spreading code demodulator) 1-48 to narrow the spectrum of an incoming signal. In FIG. 1, a code generator 1-46 generates said spreading code replica, which is correlated in a multiplier 1-50 with a received signal by said spreading code replica. If the code replica and the received code are the same and in phase, they correlate, and the transmitted data modulation can be restored to what it was before spreading. At the same time, different spurious signals are also spread. A band-pass filter 1-52, succeeding the despread modulator 1-48, lets the data modulation through, but removes most of the power of a spurious signal, which improves the signal-to-noise ratio of the received signal.
In order to enable the detection of transmitted data in a spread spectrum receiver, the code replica generated by the receiver has to be synchronized (acquisition) with the received code as accurately as possible, and said synchronization has to be maintained (signal tracking). The spreading code replica generated in the receiver thus has to be and stay in phase with the spreading code included in the received signal. For this reason, a special synchronization algorithm or unit is required for code synchronization, in addition to regular carrier and data synchronization. The speed of the acquisition, i.e. the time taken by the code replica to hit the right phase with the received code, is an important performance parameter of a spread spectrum system. Many methods have been developed for the acquisition, in addition to which the system may comprise different aids for the acquisition that are related to the transmitted signal.
Matched filters are devices whose output is a time-reversed replica, a copy of the desired incoming signal, when the input is an impulse. Thus the transfer function of a matched signal is a complex conjugate of the signal matched thereto. A matched filter can be implemented to operate either continuously or discretely. A matched filter calculates the correlation between a known reference signal and the signal to be measured, and gives a maximum output when the reference signal best corresponds to the incoming signal. For this reason, a matched filter is usable in signal acquisition in spread spectrum systems for searching for the right phase of the reference signal generated by a receiver. A matched filter may be shown to be the optimal way to identify signals from AWGN (Additive White Gaussian Noise) type of noise.
FIG. 2 shows a signal flow diagram of a feasible implementation of a matched filter. It consists of a delay line having intermediate outputs and of a passive filter matched to the waveform of a PRN (Pseudo Random Noise) chip. The output of the filter is matched to the basic pulse form of PRN spreading bits. In FIG. 2, in(n) represents a signal incoming to a filter and in(n−1), in(n−2) . . . in(n−NMF+1) represent an incoming signal delayed by 1,2 to NMF+1 delay elements Tc. c(0), c(1) . . . c(NMF−1) represent coefficients by which the incoming signal, delayed in different magnitudes, is multiplied. After the multiplication, the signals are summed up in an adder 2-10 and the sum signal is filtered in a filter 2-20.
The use of a matched filter in the synchronization of spread spectrum systems is known for example from ‘Spread Spectrum Communications Handbook’, Marvin K. Simon et al., McGraw-Hill, 1994, pages 815 to 832. In a known matched filter, the filter is matched to one received signal at a time. This requires either the use of several matched filters or the search for one signal at a time, should the intention be to search for more than one signal.
When a band-pass type of signal is searched for with a matched filter from a received noisy signal, in known solutions the signal coming to the matched filter is pre-processed by multiplying it by a carrier estimate, which removes the frequency offset of the receiver. If the frequency offset is not known, the signal has to be searched for at different frequency offsets over the entire frequency inaccuracy range. Furthermore, a matched filter searches for the right phase of the reference signal generated by a receiver. A matched filter calculates the correlation between a known signal and the signal to be measured, i.e. generates a measure for the identity of the two signals. The outputs generated by the filter are typically non-coherently detected amplitude values.
Said measure is then compared with a set threshold value in order to decide if the two signals are in sync. In the simplest case, exceeding the threshold value means that the signal corresponding to the reference signal has been identified and that the spreading code of the identified signal is in phase with the reference signal. This information serves to initiate actual signal tracking and reception. If no identification occurs (the threshold value is not exceeded), the acquisition system changes the phase of the locally generated reference code or changes reference signals, whereupon the correlation is repeated. This continues until identification and synchronization are achieved, i.e. the reference signal corresponds best to the incoming signal. In this case the filter yields a maximum output. The tracking algorithm of the received signal is then initiated.
Since in the acquisition system, a band-pass type of signal is searched, the matched filter has to be implemented either as a band-pass or an equivalent low-pass filter version. A low-pass type of acquisition system using a matched filter is shown in FIG. 3. Therein, a signal 3-1 incoming to identical, matched filters 3-10, 3-12 is divided into two parts, I and Q branches (I stands for In-phase, Q for Quadrature), and a signal that is generated by a local oscillator 3-2 and whose frequency can be substantially equal to the sum of the intermediate frequency of the receiver and the Doppler frequency of the received signal is used to multiply a signal of the 3-1 branch in a multiplier 3-6. Before a signal of the 3-Q branch is multiplied in a multiplier 3-8, the phase of a signal generated by the local oscillator is shifted 90 degrees in a phase inverter 3-4.
After the multiplication of the incoming signal, signals incoming from 3-I and 3-Q branches are correlated in substantially identical matched filters 3-10 and 3-12 with a code replica generated in the receiver. The signals outcoming from the matched signals are then detected, i.e. the signals of both branches are squared in elements 3-14 and 3-16, and the squared signals are summed up in an adder 3-18 to obtain the square of the absolute value of a complex ingoing signal. A threshold value detector 3-20 then compares the value of the detected signal with a preset threshold value, a reference value. In the simplest case, exceeding the threshold value means that a signal corresponding to said reference signal has been detected and its spreading code is in phase with the stored reference signal. The information is used to initiate actual signal tracking and reception.
In the structures of generally known matched filters, the timing of a reference signal and an incoming signal is fixed at the planning stage, and cannot thus be adjusted accurately for different timings. This causes problems for tracking signals having a low signal-to-noise ratio, since the integration time required by them is long. This, in turn, requires accurate timing in the sampling of a matched filter, since the operation of a matched filter is subject to its reference signal being of the same length as a received signal in the time domain. In systems, in which the movement of a transmitter and receiver with respect to one another is fast, a Doppler shift, whose magnitude depends on the frequency of said signal component, is created in the carrier and the spreading code. Since the frequency of the spreading code depends on the Doppler shift, the frequency is not always exactly the same. This should also be accounted for in the acquisition system if the required integration time (T1) is long. If the inaccuracy of the frequency exceeds 1/T1, the timing of the code changes more than one chip during integration, which prevents the acquisition system from operating.
The integration time of a DS-SS acquisition system is also limited by the modulation of the transmitted data. Generally, the integration cannot be continued over a transmitted data symbol unless the modulation can be compensated for before the integration. For example in the widely used BPSK modulation (Binary Phase Shift Keying), a change in a data bit causes a 180° phase change in the signal, corresponding to an inversion in its sign. This is why the integration over a data bit causes significant degradation to the signal. Consequently, when the integration time is longer than the length of a data symbol, coherent integration can no longer be used. The use of non-coherent integration only is not feasible, since non-coherent detection weakens the signal-to-noise ratio if the incoming signal-to-noise symbol is initially negative.
A pass-band or low-pass type of matched filter can be implemented either as analog or digital. The most usual way is to implement matched filters based on the analog technology, wherein the delay line is implemented by SAW (Surface Acoustic Wave) or CCD (Charge Coupled Device) technologies. However, at the manufacturing stage, said systems are built for only a given reference signal. The delay line of an analog discrete-timed matched filter can be implemented for example based on the SC technology (Switched Capacitor). However, a problem in this technology is aliasing, for example.
The advancement of the digital technology has also brought about digitally implemented matched filters. To implement the required rapid summing up of many values is difficult in a digital filter. In a matched filter, stored signal samples, multiplied by the reference signal along the length of the filter have to be calculated to generate one outcoming sample. Conventionally, this has been accomplished by summing up a small number of numbers at a time and by repeating the process during several clock cycles. This avoids the implementation of a multiple-input adder.